1. Field of the Invention
The present invention relates generally to temperature control systems, and more particularly, relates to systems to initiate temperature control of a refrigerated space.
2. Description of the Prior Art
It is known in the art to control the temperature of a space used to store or transport perishable materials such as foodstuffs. In controlling temperature in such a space, it is most difficult when the space is used to transport the perishable materials, such as with refrigeration trucks.
Much of the difficulty arises because of the change in thermal loading caused by large changes in the ambient with respect to time of day, geographical location, and weather conditions. A particular problem results from the initial loading of the refrigeration truck. It is desirable to pull down the temperature as quickly as possible to provide maximum retardation of spoilage. However, care must be exercised not to over shoot the target temperature anywhere within the refrigerated space, thereby freezing some or all of the cargo.
U.S. Pat. No. 4,715,190 discusses a rudimentary temperature control system. An advanced microprocessor based temperature control system is taught in U.S. Pat. No. 4,663,725, issued to Truckenbrod et. al., and incorporated herein by reference. The Truckenbrod et. al., system employs temperature sensors for the return and discharge of air. A fault recovery algorithm permits the system to switch between sensors as appropriate. U.S. Pat. No. 4,589,060, issued to Zinsmeyer, also discusses a microprocessor based temperature control system.
Techniques for control of the refrigeration process are described in U.S. Pat. Nos. 4,742,689 and 4,934,155, issued to Lowes. These Lowes references show control of temperature through the use of hot gas bypass of the compressor in an attempt to maintain a more constant compressor speed.
A highly sophisticated system for temperature control is taught in U.S. Pat. No. 4,918,932 issued to Gustafson et. al., and incorporated herein by reference. This system uses an integrated average of the outputs of a discharge sensor and a return sensor to control the refrigeration apparatus. The algorithm of this system treats the initial pull down of the temperature following cargo loading in the same manner as the steady state.